1. Field of the Invention
The present invention relates to optical rotation sensors based on the Sagnac effect, such as fiber optic gyroscopes. More particularly, this invention pertains to fiber optic gyroscopes that incorporate relatively low cost components to thereby minimize system cost without degrading performance.
2. Background of the Prior Art
The measurement of rotation is of considerable importance in many areas ranging from missile and aircraft guidance to spacecraft applications.
The spinning wheel or mechanical gyroscope method has been utilized for many years. Despite wide acceptance, this approach is subject to disadvantages that are known in the navigation-guidance field. These include relatively brief lifetimes due to the continuous motion of component parts and high component cost.
Optical instruments based on the Sagnac effect, such as the ring laser gyroscope, have begun to replace mechanical gyroscopes in many applications. The development of low scatter mirrors and high quality gas discharge tubes for generating laser light have hastened the development of this technology. However, the ring laser gyroscope is subject to the phenomenon of frequency-locking at low rotation rates, which can eliminate the rotation-induced frequency beat at such low rates.
The fiber optic gyroscope has emerged as an alternative to the ring laser gyro that is not subject to the problem of frequency lock-in at low rotation rates. Moreover, the fiber optic gyroscope offers the potential of reduced cost. Accordingly, this instrument may be suitable for large-volume, low and medium accuracy navigation and guidance systems.
Fiber optic gyroscopes initially employed low-birefringence fiber similar to that used in the optical fiber communications industry. A schematic diagram of a typical low-birefringence fiber optic gyroscope of the prior art is shown in FIG. 1. Such a device comprises a laser diode light source 10, two beam splitters 12 and 14 (or fiber directional couplers) to route the light, a photodetector 16, a multiturn fiber coil 18 that acts as the rotation sensing component, and a phase modulator 20 consisting of either a piezoelectric cylinder or a single-channel LiNbO.sub.3 waveguide modulator. The Sagnac phase shift is a relativistic effect that originates from a rotation-induced differential phase shift between counterpropagating waves.
One of the most notorious problems of fiber optic gyroscopes made with low birefringence fiber and low birefringence optical components is a large bias drift due to the random and unpredictable exchanges of power between the fiber's two polarization states. This bias instability, also known as polarization non-reciprocity (PNR) bias error, is briefly described below.
As described by Ezekiel and Arditty in "Fiber Optic Rotation Sensors: Tutorial Review," Proceedings of First International Conference on Fiber Optic Rotation Sensors, Cambridce, MA (1981), due to the small magnitude of the Sagnac-induced phase shift in a fiber optic gyro, spurious phase shifts caused by different parasitic effects can easily be orders of magnitude larger than the Sagnac phase shift. The only known way to cancel many of such spurious phase shifts is by employing the so-called "principle of reciprocity." According to this principle, many of such parasitic, unwanted phase shifts, regardless of size, will vanish provided that the counterpropagating waves travel identical optical paths within the sensing loop.
Single mode optical fiber actually exhibits two orthogonal polarization modes. Even though it is always possible to launch light into only one polarization mode of a low-birefringence, single mode fiber, many external birefringence perturbations (such as small lateral forces on the fiber, fiber twist or fiber bending, magnetic or electric fields) will couple power into the orthogonal mode. Consequently, in the prior art fiber optic gyroscope of FIG. 1 it is not possible to insure that the two counterpropagating waves will travel identical optical paths as portions of each wave will travel first in one polarization and then "cross-couple" to the orthogonal polarization. The principle of reciprocity is thus violated, producing polarization non-reciprocity bias (PNR) error.
According to the principle of reciprocity, the introduction of a perfect polarization filter (or polarizer) before the loop coupler (at point A in FIG. 1) will eliminate the PNR bias error. However, the introduction of a polarizer before the loop coupler in an otherwise exclusively low birefringence gyroscope will create another problem known as "polarization fading." Polarization fading is found in low birefringence fiber systems that contain polarizers. Since the state of polarization (SOP) of light in a low birefringence, single mode fiber will fluctuate (due to the previously-mentioned external birefringence perturbations that produce a continuous exchange of power between the two polarization modes), situations will occur where the SOP is orthogonal to the polarizer's transmission axis. The transmitted power in such situations is zero. Thus, large fluctuations of power (as large as 100 percent) occur over time. Polarization fading was alleviated in early fiber optic gyroscopes by periodically adjusting the SOP by hand, using fiber polarization controllers to align it with the polarizer axis. Needless to say, such a stopgap approach is inappropriate for the design of inexpensive, mass-producible devices.
In addition to polarization fading, low-birefringence fiber optic gyroscopes employing a polarizer require unrealistically high-performance polarizers to achieve satisfactory rotation rate sensitivity. It is estimated, for example, that a gyroscope comprising low-birefringence fiber and components requires a polarizer with extinction ratio of about 120 dB to attain navigation grade performance. At present, the best known polarizer has an extinction ratio of about 90 dB. The extinction ratios of low-cost, mass-producible polarizers do not currently exceed 60 to 70 dB.
The problems of polarization fading and polarization non-reciprocity bias error have been greatly alleviated by the introduction of polarization maintaining (PM) fiber and low-coherence or broadband light sources such as superluminescent diodes. See for example W. K. Burns, C. L. Chen and R. P. Moeller, "Fiber-Optic Gyroscopes with Broadband Sources," J. Lightwave Tech., Vol 1, No. 98, (1983). PM fiber is designed so that the SOP of light launched into one of the fiber polarization modes is substantially conserved over long sections of fiber (hundreds of meters or kilometers) despite the presence of external birefringence perturbations. Polarization fading basically disappears as the light's SOP is significantly aligned at all times with the polarizer transmission axis. The PNR bias error is greatly reduced because the low coherence of the light source in conjunction with the high birefringence of the PM fiber, greatly reduces the correlation between the different parasitic cross-coupled waves and the primary waves.
The use of PM fiber and components has entailed substantially increased cost in terms of both fiber and fiber component fabrication. PM fiber and PM couplers are far more costly to manufacture than their low-birefringence counterparts as the manufacture of PM couplers, for example, requires a costly alignment step.
Furthermore, a scale factor error can result if the average wavelength of the light source is not stable over changing temperatures. The average wavelength must be very stable with respect to the environment in a fiber optic gyroscope and it can be greatly affected for example by the temperature-dependence of the coupler split ratio. Prior designs have not addressed this problem in the past for systems employing PM or low-birefringence components.
Another approach that reduces both polarization fading and PNR bias error with a low birefringence fiber sensing coil is described by Ulrich in "Polarization and Depolarization in the Fiber Optic Gyroscope," First International Conference on Fiber Optic Rotation Sensors, Cambridge, Ma (1981). Ulrich discloses the principle of depolarization or polarization "averaging." According to Ulrich, if all possible SOP's of the low birefringence fiber are present during the gyroscope detector integration time, a stable polarization average is produced. A practical wa to achieve depolarization or polarization averaging is by means of a Lyot depolarizer in conjunction with a broadband light source as discussed by R. E. Epworth in "The Temporal Coherence of Various Semiconductor Light Sources Used in Optical Fiber Sensors."Ibid. Another reference on Lyot depolarizers is W. K. Burns, "Degree of Polarization in the Lyot Depolarizer," J. Lightwave Tech., Vol 1, p. 475 (1983) and such references are hereby incorporated by reference. A depolarized fiber optic gyroscope is disclosed in U.S. Pat. Ser. No. 4,828,389 of Gubbins et al. covering "Integrated Triad Optical Rate Sensor Apparatus," and the concept is further discussed in the article of J. L. Page entitled "Multiplexed Approach for the Fiber Optic Gyro Inertial Measurement Unit," SPIE Proceedings on Fiber Optic and Laser Sensors Vol 8, pgs. 93 through 102 (1990).
While the depolarized gyroscope is useful in low accuracy applications (5 to 50 deg/hr. minimum rotation sensitivity), it is questionable whether it can achieve the degree of accuracy required, for example, in inertial navigation, avionics attitude and heading reference and missile guidance applications (0.003 to 1.0 deg/hr. minimum rotation sensitivity) due to the limitations of present day polarizers and depolarizers.
SUMMARY OF THE INVENTION
The present invention overcomes the preceding shortcomings of the prior art by providing an improvement in a fiber optic gyroscope of the type that includes a light source, at least one photodetector, at least one sensing coil of optical fiber and at least one modulator. The improvement in such apparatus provided by the gyroscope of the present invention lies in the utilization of both low-birefringence and polarization maintaining (PM) components, the sensing coil being of PM fiber composition.
The foregoing and additional features and advantages of the present invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing FIGURES. Numerals of the drawing FIGURES, corresponding to those of the written description, point to the features of the invention. Like numerals refer to like features throughout both the drawings and the written description.